The control of tension in load bearing lines is required in many different circumstances. The desired nature of the control varies according to the circumstances. Often it is considered desirable for the tension to be progressively increased as the connection made by the line is elongated. Methods are presently available for producing such a pattern of control. For instance, a heavy catenary line provides progressively greater tension as it is stretched until it becomes bar-taught. Pneumatic spring devices are known which provide a similar increase in tension with increasing excursion. For instance, German specification No. 54186 discloses a device comprising a cylinder and a piston for mounting on a vessel connected to the anchor chain, the cylinder being in fluid connection with a reservoir. The cylinder and part of the reservoir contain liquid and the remainder of the reservoir contains a pressurised gas which is gradually further compressed upon the vessel moving away from its anchor. Such an arrangement provides increasing tension with excursion of the vessel from its mooring point.
Essentially similar devices are disclosed in Dutch patent specification No. 7312778, Dutch patent specification No. 7808618 and European patent application No. 0045652.
There are a variety of other circumstances however in which it is desirable to provide a different pattern of variation of tension in a line with varying degrees of excursion of the objects connected by the line. For instance, it has now been discovered that in deep sea anchorages the use of a rising rate type of tension device such as a heavy catenary line or a pneumatic device of the kind shown in German patent specification No. 54186 leads to undesirable results. In particular, the normal load in the line is excessive and is significantly above that actually required on average.
Moreover, the maximum load experienced in the line is very heavily dependent upon the maximum excursion experienced and a miscalculation of the excursion to be expected could lead to very much higher loads being experienced in the line than expected, with consequent difficulties such as parting of the line or dragging of anchors.
Furthermore, the use of conventional mooring systems provides other disadvantages such as the long distance to anchors necessary with multiple catenary moorings which imposes limitations on the disposition of the anchors having regard to sea bed obstructions such as sea bed equipment. In the case of the use of spring buoys as tension control devices in moorings, the amount of buoyancy required in the spring buoy to provide a strong enough spring is sometimes so large that major structures are required on the sea bed to take the additional uplift force generated by the buoyancy of the buoy and furthermore, providing the required buoyancy may entail large buoyant structures which themselves will, even when submerged, attract wave forces which will be additional to the forces imposed by the moored structure itself.
It is accordingly desirable to provide devices for controlling the tension in lines such as mooring lines which provide a different variation of tension with excursion than the systems described above or which avoid the use of large buoyant structures as a means of tension control.
In yet other circumstances, it is desirable to be able to alter the pattern of tension variation with excursion to fit the particular circumstances in which the equipment is being used.
British patent specification No. 849887 discloses an anchoring system in which excursion of a moored platform is controlled by lines connected to weights so that there is a constant force in the line despite excursion of the platform or in an alternative embodiment the lines are connected to pneumatic cylinders working against a constant pressure so that again there is constant tension in the lines. However, the apparatus described in specification No. 849887 is not adapted for use in other circumstances than the particular type of structure shown. In particular, it is not adapted for use at an intermediate position in a line connecting two relatively moveable objects.